Please ignore this post. Problem solved.
Using Office 2007/ Outlook. I have been happily downloading emails from two very similar sources (imap) for years, and suddenly Outlook fails to list or download messages that are more recent than March of this year from one of accounts . I know that there are emails to see, as I can log on to the source account and find them, but they are just not coming through to Outlook. The other account works fine.
Any theories?